Implement shop from its specification

shop in src/freight/shop.py is declared, documented and tested, but its

body raises NotImplementedError. Write it.

The full contract is in the module's docstring: which carriers get asked,

why the others do not, how billable weight is computed per carrier, the

order of the surcharge pass and how a group decides between its members,

what a rejection looks like, and how the cheapest quote is chosen when the

prices tie. Read it before writing a line of code; it is the specification,

and the evaluation measures against it.

The short version

• Only carriers that hold a card for the band of the lane are quoted; the

billable weight is the card's own divisor, minimum and step.

• The line-haul comes from the card. Surcharges run in declaration order,

each one against the quote as it looks at that moment; a group keeps its

most expensive member and suppresses the rest, naming the winner.

• A card that refuses the weight, or a carrier with no card for the band,

is a rejection with a reason, not a price.

• The cheapest total wins; ties go to the lower billable weight, then the

alphabetical carrier name. A run with no price at all is not an empty

result: it raises NoQuotableCarrier.

Boundaries

• Implement it in src/freight/shop.py; the public API of freight stays

as it is, and nothing else in the package changes.

• The function is pure: it reads the shipment and the tables and returns a

ShoppingResult; calling it twice on the same input returns the same

answer, in the same order.

• Do not add third-party dependencies — the standard library is enough.
• The repository's own test suite (pytest) has to stay green.